Simulated time-dependent climate response to solar radiative forcing since1600

Estimated solar irradiance variations since 1500 have been used to force th e GISS atmospheric GCM coupled to a mixed layer "q-flux" ocean with heat di ffusion through the bottom of the mixed layer. The goal is to assess solar- induced climate change in preindustrial and postindustrial epochs. Six simu lations and control runs were made to test the effects of different initial conditions, estimates of initial solar forcing conditions, and ocean heat uptake. The results show that an estimated solar forcing increase of 0.25% accounts for a 0.45 degrees C temperature increase since 1600 and an increa se of about 0.2 degrees C over the past 100 years. Global surface temperatu res lag solar fluctuations by up to 10 years; the lag is greater over the o ceans and so is the correlation due to reduced noise. With only a mixed lay er ocean the phase lag is 5 years less. Solar forcing and water vapor feedb ack each directly account for 35% of the temperature response, with cloud c over changes contributing 20% and sea ice/snow cover 10%. Uncertainty in th e initial radiation imbalance or solar forcing affects the surface temperat ures for 60-90 years. Modeled and observed periodicities show dominance of long-period forcing (>50 years), as provided by the solar input in these ex periments. Tropical temperatures correlate best with solar forcing, due to the influence of water vapor feedback, especially at these multidecadal per iods. Sea ice and extratropical temperatures have less long-period power, w hile high-frequency fluctuations dominate simulated cloud cover variations, which are relatively independent of solar forcing changes. Global and extr atropical precipitation increase as the climate warms, but not low and subt ropical precipitation, due to conflicting influences of absolute temperatur e and temperature gradient changes. Solar forcing by itself was not suffici ent to produce the rapid warming during the last several decades. A compari son experiment varying trace gas forcing suggests that if the solar estimat e is correct, then negative forcing by tropospheric aerosols (and perhaps v olcanoes, ozone, and land use changes) has been about -1.2 W m(-)2 since 17 00, implying approximately equal contribution from direct and indirect trop ospheric aerosol effects.